The invention relates to an optical transmission system comprising an optical transmitter coupled via a glass fibre to at least an optical receiver, the optical transmitter comprising an electro-optical converter and linearization means for linearizing the relation between an electric input signal of the electro-optical converter and a parameter of an optical output signal of the electro-optical converter. The invention likewise relates to an optical transmitter to be used in such a transmission system.
A transmission system as defined in the opening paragraph is known from the journal article "Comparison of direct and external modulation for CATV Lightwave transmission at 1.5 .mu.m wavelength" in Electronics Letters 24, September 1992, Vol. 28, No. 20 pp. 1875-1876.
In the known transmission system an electrical input signal is applied to an electro-optical converter which converts the input signal into changes of a parameter of the light emitted by the electro-optical converter. This parameter may be, for example, the intensity (power), frequency, phase or a combination of these parameters. The electro-optical converter may comprise, for example, a laser or LED directly controlled by the input signal. Alternatively, it is conceivable that the electro-optical converter comprises a laser or LED which continously generates a light signal that is modulated by an external modulator which is controlled by the input signal.
For example, the input signal may be a signal in one of a combination of a large number of TV channels transmitted in the frequency-division multiplex mode, such as is customary in cable television systems. When such signals are transmitted, very strict requirements are made on the linearity of the relation between the input signal and the corresponding parameter of the optical output signal of the electro-optical converter. This is essential, because intermodulation in the transmission system may occur as a result of non-linearity, so that an (undesired) mixing product of two TV channels may cause an interference signal to occur in a third TV channel.
In the known transmission system linearization means to improve this linearity are used which effect a predistortion of the input signal that is the reverse of the distortion occurring in the electro-optical converter. As a result, a more linear relation between input signal and the corresponding parameter of the optical output signal of the electro-optical converter is obtained. The linearization means in the known transmission system are rather complex, however, due to the presence of a power divider, a predistortion circuit, a controllable delay and a power combiner.